[da-x]

I wonder didn't they used VDO thin provisioning with LVM2.

Also, a few years ago I've implemented VM management tool called 'vmess', in which the concept is to maintain a tree of QCOW2 files, which R/W snapshots are at the leafs and R/O snapshots are the nodes of the tree. The connection up to the root is made via QCOW2 backing-file store mechanism, so a newly created leaf starts a 0 space. I did this because libvirt+qemu impose various annoying limitations surrounding snapshots-with-in-qcow2, and I liked the idea of file-per-snapshot.

VDO: https://docs.kernel.org/admin-guide/device-mapper/vdo.html (original project URL: https://github.com/dm-vdo/kvdo )

vmess: https://github.com/da-x/vmess